An operating system views a processor as a logical processor. This logical processor is backed by a certain physical processor. The physical assignment of processors is determined when the machine is powered on and initialized before the operating system is loaded. Normally, once the assignment of logical to physical processor is made, this assignment is not modified.
However, under certain stringent conditions, the assignment of logical to physical processor can be modified. In particular, if a processor fails, the assignment can be modified, assuming a spare processor exists within the processing environment. Specifically, in situations of failed processors, the assignment is modified, such that the spare processor, instead of the failed processor, backs the logical processor.